This invention relates generally to golf balls, and more specifically, to a multilayer golf ball having improved adhesion between adjacent component layers as well as method for forming such golf balls. In particular, this invention relates to a golf ball having a core, an inner cover layer and a very thin outer cover layer, wherein the outer cover layer comprises a thermoset material formed from a castable, reactive liquid and the inner cover layer comprises a high flexural modulus material with a textured pattern to improve adhesion between the layers.
Conventional golf balls can be divided into three general types or groups: (1) two piece balls, (2) wound balls (also know as three piece balls), and (3) multilayer balls. The difference in play characteristics resulting from these different types of constructions can be quite significant.
Balls having a two piece construction are generally most popular with the recreational golfer because they provide a very durable ball while also providing maximum distance. Two piece balls are made with a single solid core, usually formed of a crosslinked rubber, which is encased by a cover material. Typically the solid core is made of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents. The cover comprises tough, cut-proof blends of one or more materials known as ionomers such as SURLYNs(copyright), which are resins sold commercially by DuPont or lotek(copyright) which is sold commercially by Exxon.
The combination of the above-described core and cover materials provides a xe2x80x9chardxe2x80x9d covered ball that is resistant to cutting and other damage caused by striking the ball with a golf club. Further, such a combination imparts a high initial velocity to the ball which results in increased distance. Due to their hardness however, these balls have a relatively low spin rate which makes them difficult to control, particularly on shorter approach shots. As such, these types of balls are generally considered to be xe2x80x9cdistancexe2x80x9d balls. Because these materials are very rigid, many two piece balls have a hard xe2x80x9cfeelxe2x80x9d when struck with a club. Softer cover materials such as balata and softer ionomers in some instances, have been employed in two piece construction balls in order to provide improved xe2x80x9cfeelxe2x80x9d and increased spin rates.
Wound balls typically have either a solid rubber or liquid filled center around which many yards of a stretched elastic thread or yarn are wound to form a core. The wound core is then covered with a durable cover material such as a SURLYN(copyright) or similar material or a softer cover such as balata or polyurethane. Wound balls are generally softer than two piece balls and provide more spin, which enables a skilled golfer to have more control over the ball""s flight and final position. In particular, it is desirable that a golfer be able to impart back spin to a golf ball for purposes of controlling its flight and controlling the action of the ball upon landing on the ground. For example, substantial back spin will make the ball stop once it strikes the landing surface instead of bounding forward. The ability to impart back spin onto a golf ball is related to the extent to which the golf ball cover deforms when it is struck with a golf club. Because wound balls are traditionally more deformable than conventional two piece balls, it is easier to impart spin to wound balls. However, wound higher spinning balls typically travel a shorter distance when struck as compared to a two piece ball. Moreover, as a result of their more complex structure, wound balls generally require a longer time to manufacture and are more expensive to produce than a two piece ball.
The United States Golf Association (USGA) has instituted a rule that prohibits the competitive use in any USGA sanctioned event of a golf ball that can achieve an initial velocity of greater than 76.2 meters per second (m/s), or 250 ft/s, when struck by a driver with a velocity of 39.6 m/s, i.e., 130 ft/s (referred to hereinafter as xe2x80x9cthe USGA testxe2x80x9d). However, an allowed tolerance of two percent permits manufacturers to produce golf balls that achieve an initial velocity of 77.7 m/s (255 ft/s).
Regardless of the form of the ball, players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact. Therefore, in an effort to meet the demands of the marketplace, manufacturers strive to produce golf balls with initial velocities in the USGA test that approximate the USGA maximum of 77.7 m/s or 255 ft/s as closely as possible.
Therefore, golf ball manufacturers are continually searching for new ways in which to provide golf balls that deliver the maximum performance in terms of both distance and spin rate for golfers of all skill levels.
Relatively recently, a number of golf ball manufacturers have introduced multilayer golf balls, i.e., having multiple core intermediate mantle and/or cover layers, in an effort to overcome some of the undesirable aspects of conventional two piece balls, such as their hard feel, while maintaining the positive attributes of these golf balls (including their increased initial velocity and distance). Further, it is desirable that such multilayer balls have a xe2x80x9cclick and feelxe2x80x9d as well as spin characteristics approaching that of wound balls.
Multilayer golf balls can be formed using a variety of constructions. For example, multilayer balls may have two or more cover layers molded around a conventional core with one or more intermediate layers interposed between the cover and the core. Likewise, multilayer balls may be formed from cores having more than one core layer and may optionally contain one or more intermediate and/or cover layers. Multilayer balls may even comprise a conventional wound core around which at least one intermediate layer and/or at least one cover layer is formed. Examples of multilayer balls include the Altus Newing (Bridgestone), Reygrande 2xc3x972 (Bridgestone), Giga (Spalding) Metal Mix (Dunlop), Ultra Tour Balata (Wilson).
Typically, the layers of multilayer golf balls are formed by molding them around the core or a preceding intermediate layer or cover layer. Conventional techniques for applying such layers include injection molding, compression molding and casting the layer material around the preceding core or layer. Accordingly, a crucial aspect of the manufacture of multilayer balls is obtaining good adhesion between the various layers. If the adhesion between the layers does not meet desired standards, the performance or durability of the golf ball will be adversely affected. For example, poor adhesion can cause air pockets between the layers which can result in separation of the layers when the ball is struck with a club.
It is well known that the adhesion between the wound core and the cover of a wound ball is enhanced due to the small imperfections created in the uneven outer surface of the winding formed by overlapping the thread. Cover material flows into these imperfections when the cover is molded about the wound core, resulting in improved adhesion. In contrast, however, in two piece and multilayer balls, adhesion between the core, cover and/or intermediate layers is greatly reduced due to the relatively smooth outer surface of the layers.
Accordingly, there are a number of methods known in the art directed towards promoting adhesion between the various solid layers of a golf ball. For example, U.S. Pat. Nos. 4,229,401 and 4,173,345 are directed towards alleviating problems associated with compression molding covers about a core by providing a series of surface channels {fraction (1/16)} of an inch deep which encircle the outer surface of the core and pass through both polar areas thereof. Providing such channels was found to prevent separation of the cover from the core when the ball is struck with a golf club by eliminating the entrapment of air between the cover and the core during the compression molding process.
Additionally, there are a number of patents directed towards promoting adhesion between core layers and covers, as well as adhesion between intermediate layers, including wound layers, through the use of projections or prongs extending from the outer surface of the core or an intermediate layer. For example, U.S. Pat. Nos. 721,852 and 2,229,170 disclose the use of a plurality of uniform sized projections extending from the surface of the core. Such projections are designed to provide an anchoring means for an elastic thread winding.
Moreover, U.S. Pat. No. 697,925 discloses game balls wherein a metallic core is covered by a soft rubber envelope which contains a plurality of deep xe2x80x9cpitsxe2x80x9d extending about halfway through the envelope. A hard shell material is compression molded around the core/envelope such that the shell material flows into the pits, thereby forming prongs which interlock the hard shell with the soft rubber envelope.
Additionally, it is known in the golf ball art that adhesion between the cover and the core of a two-piece golf ball can be improved by roughening the outer surface of the core layer. For example, U.S. Pat. No. 1,558,706 provides a very general disclosure of improving the adhesion of a two piece ball having a vulcanized rubber core and a balata shell, wherein the outer surface of the core is roughened by forming corrugations which assist in interlocking the cover and the core so as to prevent any movement therebetween when the ball is struck with a club. Although the ""706 patent discloses the general concept of roughening the core surface to improve adhesion, it does not provide any details as to how the core surface is roughened nor any specifics regarding the characteristics of the corrugations in the core surface.
Similarly, U.S. Pat. No. 4,367,873 discloses making a softball or a baseball having a PVC foam core and an ethylene copolymer core cover, wherein the core cover has corrugations one eighth to one sixteenth of an inch high. A layer of yarn windings is wound around the core cover and a leather outer cover is then placed around the windings. The ""873 patent discloses that such corrugations may have a buffering effect when the ball is impacted and provide a more resilient ball as well as possibly increasing the binding friction between the outer leather cover and the inner core.
One conventional method for improving the adhesion between the layers of a multilayer golf ball is to texture the outer surface of, e.g., a core or a core with an intermediate layer(s) molded around it by manually roughening or scuffing the outer surface after molding and prior to forming the subsequent layer thereon. Such surface roughening is typically obtained by grinding the surface with a grinder such as a Glebar grinder or by tumbling or milling the ball at various stages of its construction in an abrasive media. Likewise, the surface may be roughened by blasting the surface of the layer with sand or some other abrasive material.
After the surface is roughened, it must be washed to remove any loose material or excess abrasive material. Otherwise, the loose material and/or excess abrasive material which remains on the surface will form air pockets when the next layer is molded thereon. As mentioned above, such air pockets are undesirable as they lead to separation of the layers. Further, the roughened surface must be completely dried before the next layer can be applied. If it is not dried completely, the remaining moisture is converted into steam when the next layer is molded around it, forming defects in the interfacial bonding area that can also lead to separation of the layers.
Although such post-molding methods roughen the outer surface of the layer, it is difficult to obtain sufficient texture depths with these processes. Moreover, even after washing and drying the treated surface, there is a tendency for small particles from the golf ball layers and/or excess abrasive media to remain in the interfacial bonding area between the layers which are easily dislodged when the ball is struck with a club and can form air pockets between the layers which can lead to the separation of the layers. Similarly, inadequate texture depth can also lead to separation of the layers.
Further, such post-molding roughening methods require, at a minimum, three distinct steps: (1) roughening the surface via grinding; (2) washing the roughened surface to remove excess debris; and (3) thoroughly drying the roughened surface before the next layer can be applied. Each of these process steps are labor intensive, time-consuming and add additional expense to the production of multilayer golf balls.
Thus, there remains a need for an efficient method for improving the interfacial adhesion between golf ball layers, wherein the outer surface of a layer is provided with a texture of an adequate depth and free of debris, while eliminating any post-molding treatments such as grinding, sandblasting, washing and drying.
Additionally, a number of patents have been issued directed towards modifying the properties of a conventional two piece ball by altering the typical single layer core and/or single cover layer construction to provide a multilayer core and/or cover. The inventions disclosed in these patents are directed towards improving a variety of golf ball characteristics.
For example, there are a number of multilayer ball patents directed towards improving the spin, click or feel of solid balls while maintaining the distance provided by the solid construction. A variety of approaches to manipulating the core construction are described in the art. For example, U.S. Pat. No. 5,072,944 discloses a three-piece solid golf ball having a center and outer layer which are prepared from a rubber composition, preferably having a base rubber of polybutadiene. This patent teaches that it is desirable that the center core is softer than the outer layer, wherein the layers have a hardness (Shore C) of 25-50 and 70-90 respectively.
U.S. Pat. No. 4,625,964 relates to a solid golf ball having a polybutadiene rubber core of a diameter not more than 32 mm, and a polybutadiene rubber intermediate layer having a specific gravity lower than that of the core material.
U.S. Pat. No. 4,848,770 discloses a non-wound three-piece golf ball which includes a core of a highly filled synthetic rubber or polymeric material, an intermediate mantle of an unfilled synthetic rubber and a cover. The core and intermediate mantle have a hardness between 50-95.
U.S. Pat. No. 5,002,281 is directed towards a three-piece solid golf ball which has an inner core having a hardness of 25-70 (Shore C) and an outer shell having a hardness of 80-95 (Shore C), wherein the specific gravity of the inner core must be greater than 1.0, but less than or equal to that of the outer shell, which must be less than 1.3.
U.S. Pat. No. 5,253,871 concerns a golf ball having a three piece structure comprising an elastomer core, an intermediate layer of a thermoplastic material containing at least 10% of ether block copolymer, preferably blended with an ionomer and a thermoplastic cover.
Several additional patents are directed to golf balls having multiple cover layers. For example U.S. Pat. No. 4,431,193 relates to a golf ball having a multilayer cover wherein the inner layer is a hard, high flexural modulus ionomer resin and the outer layer is a soft, low flexural modulus ionomer resin, wherein either or both layers may comprise a foamed ionomer resin.
U.S. Pat. No. 5,314,187 also relates to golf balls having a cover formed with multiple layers, wherein the outer layer is molded over the inner layer and comprises a blend of balata and an elastomer and the inner layer is an ionomer resin.
U.S. Pat. No. 4,919,434 is directed towards a golf ball having a cover which comprises an inner layer and an outer layer each of which comprise a thermoplastic resin, preferably the layers comprise materials that are capable of fusion bonding with each other.
UK Patent Application Nos. GB 2,291,817 and 2,291,812 are both directed towards a wound golf ball with improved distance comprising a dual cover layer, wherein the inner cover layer has a high hardness as compared to the outer cover layer. These references teach that the cover layers may be formed from balata or ionomer resins and should have a combined thickness of less than 4 mm.
UK Patent Application No. GB 2,278,609 discloses a multilayer golf ball providing enhanced distance without sacrificing playability or durability comprising a core, an inner cover layer and an outer cover layer wherein the inner cover layer comprises a high acid ionomer and the outer cover layer comprises a soft ionomer or a non-ionomeric thermoplastic elastomer.
However, none of these patents disclose a multilayer ball having a very thin thermoset outer layer formed from a castable reactive liquid as disclosed herein to provide golf balls exhibiting a xe2x80x9cprogressive performancexe2x80x9d such as those golf balls of the present invention.
The present invention is directed towards a multilayer golf ball having improved adhesion between adjacent component layers as well as methods for forming such golf balls. The cover of such golf balls are comprised of an inner layer and a cast outer layer.
The present invention is further directed towards a multilayer golf ball which in general comprises a core, an inner cover layer and a very thin (i.e.,  less than 0.05xe2x80x3) outer cover layer, wherein the inner cover layer comprises a high flexural modulus material and the outer cover layer comprises a thermoset material which is formed from a castable reactive liquid material.
In one particular embodiment, the present invention is directed towards a multilayer golf ball which comprises a core, an inner cover layer and a very thin outer cover layer, wherein: a) the core comprises a solid or liquid filled center around which a length of elastic thread is wound; b) the inner cover layer comprises a high flexural modulus material; and c) the outer cover layer comprises a thermoset material which is formed from a castable reactive liquid.
The present invention also is directed towards a method for improving the interfacial adhesion between the layers of golf balls, and multilayer golf balls in particular, as well as golf balls made from such methods.
In particular, the present invention is directed to a golf ball having improved interfacial adhesion between the layers forming the ball, wherein the ball comprises at least a cover and a core. The ball comprises a textured pattern on the exterior surface of the core, said pattern comprising a plurality of peaks having a height of about 2 to 15 mils and a draft angle of less than about 10 degrees. A cover material covers the textured core to form a two-piece ball.
In an alternate embodiment, the present invention further includes a golf ball comprised of one of more layers for the cover, wherein the inner layer(s) has a textured surface profile integrally formed in its outermost surface. The inner layer is formed by placing a core into a mold cavity, wherein the cavity has a textured pattern on the interior surface thereof. The pattern comprises a plurality of depressions having a depth of about 2 to 15 mils and a draft angle of less than about 10 degrees. The inner layer is preferably injection molded or compression molded such that an inner layer is formed with a textured surface profile on its outermost surface which is a corresponding image of the pattern of the mold cavity and comprises a plurality of peaks having heights of about 2 to about 15 mils. An outer cover material is subsequently cast over the textured intermediate layer.
The present invention is also directed to a golf ball having at least a cover, a core and optionally one or more intermediate layers disposed between the cover and the core, wherein the core and/or at least one intermediate layer has a textured surface profile integrally formed on its outermost surface. The textured surface profile comprises a plurality of peaks having heights of about 2 to about 15 mils. The textured surface profile may be a repeating pattern or it may be a random pattern of peaks. Such patterns may have peaks of a regular geometric shape or an irregular shape. The outer cover layer is cast over the core or intermediate layer and has a thickness of less than about 0.05 inches.
Additionally, the present invention is further directed towards a mold for forming a golf ball wherein the mold comprises a cavity having a textured pattern on the interior surface thereof, said textured surface comprising a plurality of depressions having depths of about 2 mils to about 15 mils and preferably a draft angle of less than about 10 degrees.